A block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, which has a relatively high vinyl aromatic hydrocarbon content, has been used for injection molding applications and extrusion molding applications such as sheets and films, utilizing characteristics such as transparency and impact resistance. In particular, a heat shrinkable film using the block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene has no problems of a residual monomer, plasticizer residues and the generation of hydrogen chloride in burning of a vinyl chloride resin which has conventionally used, so that it has been utilized for food packaging, cap seals, labels and the like. As characteristics necessary for a heat shrinkable film, there are requirements such as natural shrinkability, low-temperature shrinkability, transparency, mechanical strength and aptitude for packaging machinery. In order to improve these characteristics and obtain a good balance of physical properties, various studies have hitherto been made.
For example, the document 1 identified below discloses a method for producing a heat shrinkable film by preheating a styrene-butadiene block copolymer, and then, stretching it, in order to improve heat shrinkability. The document 2 identified below discloses a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid derivative copolymer having an aliphatic unsaturated carboxylic acid derivative content of 5 to 80% by weight and a Vicat softening point not exceeding 90° C., and a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a composition excellent in mechanical characteristics, optical characteristics, stretching characteristics, crack resistance characteristics and the like. The document 3 identified below discloses a heat shrinkable film which comprises a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, the segments thereof having a specific Tg, in order to obtain a heat shrinkable film excellent in shrinkable characteristics and environmental destruction resistance. The document 4 identified below discloses a low-temperature shrinkable film obtained by stretching a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid derivative copolymer having a vinyl aromatic hydrocarbon content of 95 to 20% by weight and a Vicat softening point not exceeding 90° C., and a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain low-temperature shrinkability, optical characteristics, crack resistance characteristics, dimensional stability and the like. The document 5 identified below discloses a polystyrene heat shrinkable film comprising a composition of a block copolymer comprising a styrene-based hydrocarbon and a conjugated diene hydrocarbon and a styrene-based hydrocarbon-containing random copolymer having a specific Tg, in order to improve natural shrinkability at room temperature. The document 6 identified below discloses a transparent high-strength resin composition containing a block copolymer comprising a vinyl aromatic hydrocarbon block of a specific structure and a copolymer block of a vinyl aromatic hydrocarbon and a conjugated diene, and a copolymer of a vinyl aromatic hydrocarbon and a (meth)acrylic ester, in order to obtain a resin composition excellent in transparency and impact resistance. The document 7 identified below discloses a multilayer low-temperature shrinkable film having at least one layer of a composition of a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic acid derivative copolymer having a vinyl aromatic hydrocarbon content of 95 to 20% by weight and a Vicat softening point not exceeding 90° C. and a block copolymer comprising a vinyl aromatic hydrocarbon and a conjugated diene, in order to obtain a shrinkable film excellent in low-temperature shrinkability, optical characteristics, crack resistance characteristics, dimensional stability and the like. The document 8 identified below discloses a multilayer low-temperature shrinkable polystyrene film having a specific heat shrinkage ratio in which an intermediate layer contains a styrene-(meth)acrylic ester copolymer having a specific Vicat softening point as a main component, and inner and outer layers have a styrene-conjugated diene block copolymer having a specific Vicat softening point as a main component, in order to obtain a heat shrinkable film excellent in heat shrinkability at low temperatures, shrinkage finishing properties and natural shrinkage ratio, and developing no blocking between films in a hot state.
However, these block copolymers comprising a vinyl aromatic hydrocarbon and a conjugated diene, or the compositions comprising the block copolymer and a vinyl aromatic hydrocarbon-aliphatic unsaturated carboxylic derivative copolymer are insufficient in a balance of natural shrinkability, low-temperature shrinkability, rigidity, transparency, blocking resistance, resistance to fusion bonding in hot water and impact resistance, and in inhibition of fish eyes (FE's) caused by gels. These documents do not disclose any methods for improving them.
[Patent Document 1]                JP 57-34921 A        
[Patent Document 2]                JP 59-221348 A        
[Patent Document 3]                JP 60-224520 A        
[Patent Document 4]                JP 61-25919 A        
[Patent Document 5]                JP 4-52129 A        
[Patent Document 6]                JP 7-216187 A        
[Patent Document 7]                JP 61-41544 A        
[Patent Document 8]                JP 2002-46231 A        
An object of the present invention is to provide a block copolymer and a hydrogenated product thereof, suitable for a heat shrinkable film, which is excellent in a balance of physical properties such as natural shrinkability, low-temperature shrinkability, rigidity, transparency and impact resistance, and further to provide a composition thereof. Further, another object of the invention is to provide a sheet/film, a heat shrinkable film and a heat shrinkable multilayer film which are excellent in a balance of blocking resistance and resistance to fusion bonding in hot water, in addition to the above-mentioned characteristics, and have a few FE's.